The latest figures from the Office of Budget Responsibility suggest North Sea tax revenues are on a downward trend, and that oil’s importance to the UK economy is declining.

Oil and gas currently account for 0.4 per cent of GDP - down from 0.7 per cent last year - and the figure is expected to plunge to just 0.03 per cent by 2040, according to the fiscal watchdog.

Danny Alexander, Chief Secretary to the Treasury, said the figures confirmed the Scottish Government’s “secret internal analysis” of the situation, revealed earlier this year in a leaked memo, despite public predictions of another boom.

He added: “This would leave a massive gap in an independent Scotland's finances.

“Scotland is better off dealing with a volatile resource like oil in the UK, where together we can provide the long-term certainty on decommissioning relief that is helping to bring forward investment right now, while managing both the fluctuations and the projected decline in revenues."

Alistair Darling, the ex-chancellor and leader of the Better Together campaign, said the analysis made clear that North Sea revenues made an important, but declining, contribution to the economy.

He added: “The oil and gas industry is good for Scotland. It provides thousands of jobs and generates revenue to pay for our public services.

“Scotland and the rest of the UK benefit from the sharing of energy resources, risks and rewards. There is no sense in putting this at risk.

“It is absolute madness for the SNP to base their case for separation around a commodity that is declining and volatile.

“Alex Salmond must explain to the Scottish people how all his promises would be funded if his forecasts turn out to be wrong.

“We know that privately the SNP accept the OBR’s figures, yet in public they cook the books and predict an oil boom.”

In March, the Scottish Government published its own forecasts for future revenues.

In its forecast the OBR predicts revenues of £33.2 billion up to 2017/18, compared to three possible scenarios outlined by the Scottish Government of 41.5, 48.1 and 57.1 billion pounds.

The Scottish Conservatives said the new figures were a huge blow to the SNP’s main economic argument.

Murdo Fraser, the Scottish Conservative MSP and convenor of the Scottish Parliament’s energy committee, said an independent Scotland would have a “huge black hole to fill to pay for public services, even based on the most optimistic of SNP calculations”.

He added: “This is a devastating blow for the SNP’s independence case. In the past it has accepted the credibility of the OBR forecasts.

“Now these very figures show, even on the most careful of projections, there would be an £8.3 billion black hole in the finances of an independent Scotland, compared to what the Scottish Government promises.

“When more optimistic forecasts are considered that gap increases to £23.9 billion.”

However, a spokesman for the Scottish Government insisted the sector was going from “strength to strength”, with record levels of investment in the North Sea projected to reach £13 billion in 2013.

He added: "Indeed, this has led to the UK Government itself describing the latest North Sea oil and gas licensing round as a 'bonanza'.

"There are estimated to be up to 24 billion barrels of oil remaining with a potential wholesale value of £1.5 trillion remaining in the North Sea which means, by value, more than half of North Sea oil and gas resources have still be to be extracted.

"The OBR's central forecast is cautious and relies on both production and price forecasts below the levels assumed by industry and other independent bodies.

"The report shows that adopting the latest industry production forecasts could boost future tax revenues by £17.4 billion above the OBR's central forecast whilst assuming prices follow a path similar to that assumed by the International Energy Agency would boost future receipts by £25.9 billion."